I found some emeralds earlier and apparently they can't be sold to the server. What are they generally used for?
You can use emeralds as building decorations, you can sell them to player shops found in the "market" portal at /spawn for about 37-45 each. You can use them to trade with villagers in the mining world if you're lucky enough to find a villager! You can buy villagers too but there is always a risk that they won't have good trades when you spawn them in the mining world.
It is a purely player market for emeralds, which is very nice for a change since you can sell almost anything else. Adds a little spice to the market
Sooner or later, I will need to be buying these for a HUGE project (involves spawn) so stock up and I'll see if we can come to a price!